C sp 3H Bond Activation Triggered by Formation of Metallacycles: Rhodium(I)-Catalyzed Cyclopropanation/Cyclization of Allenynes

Cobalt-Catalyzed Chemo- and Stereoselective Arylative Carbocyclization of 1,6-Allenynes

Different from the well-investigated enynes, transition-metal-catalyzed carbocyclization reactions of allenynes are more attractive as a result of the unique structure and versatile reactivity of allenes. Herein, we report the first cobalt-catalyzed highly chemo- and stereoselective arylative carbocyclization of 1,6-allenynes with arylboronic acids, affording five-membered carbocycles and heterocycles with moderate to high yields, broad substrate scope, and wide functional group compatibility. Moreover, several mechanistic experiments were conducted to gain insight into the reaction process.

Cobalt(I)-Catalyzed Reductive Cyclization of Enynes and Diynes Using Hydrogen Gas as a Reductant

Reductive cyclization of enynes and diynes by using H₂ gas as a reductant was realized, and the corresponding cyclized products were obtained in good yields without olefin isomerization and over-reduction of the products. By an experiment using D₂ instead of H₂, it was confirmed that H₂ unambiguously operates as a reductant in this reaction. The protocol of the reaction is very economical and user-friendly, using air- and moisture-stable CoBr₂·6H₂O and 1 atm of pressure of H₂.

Ir-Catalyzed cyclization of α,ω-dienes with an N-methyl group via two C-H activation steps

Iridium-catalyzed sp³ C-H alkylation of an N-methyl group with 1,5- and 1,6-dienes proceeded to give five- and six-membered carbocyclic compounds, respectively, in high yields. The reaction involves intermolecular alkylation of the N-methyl group with a vinyl moiety and subsequent intramolecular cyclization at the β-position of the initially formed alkylated intermediate. The reaction using a chiral bidentate phosphine ligand enabled the asymmetric synthesis of the cyclic compounds.

Hypervalent iodine promoted the synthesis of cycloheptatrienes and cyclopropanes

A new strategy is reported for intramolecular Buchner-type reactions using PIDA as a promotor. Traditionally, the Buchner reaction is achieved via Rh-carbenoids derived from Rh^II catalysts with diazo compounds. Herein, the first metal-free Buchner-type reaction to construct highly strained cycloheptatriene- and cyclopropane-fused lactams is presented. The advantage of these transformations is in their mild reaction conditions, simple operation, broad functional group compatibility and rapid synthetic protocol. In addition, scaled-up experiments and a series of follow-up synthetic procedures were performed to clarify the flexibility and practicability of this method. DFT calculations were carried out to clarify the mechanism.

A new strategy is reported for intramolecular Buchner-type reactions using PIDA as a promotor.

A Rare Alder-ene Cycloisomerisation of 1,6-Allenynes

Thermally induced cycloisomerization reactions of 1,6-allenynes gives α-methylene-γ-lactams via intramolecular Alder-ene reactions. The mechanism is supported by computational and deuterium labeling studies. This thermal, non-radical method enables the discovery of a hitherto unknown route that proceeds via a divergent mechanism distinct from the previous [2+2] cycloisomerization manifold.

Rhodium(I)-Catalyzed Enantioselective Cyclization of Enynes through Site-Selective C(sp3)–H Bond Activation Triggered by Formation of Rhodacycle

Rhodium(I)-catalyzed enantioselective cyclization of enynes through C(sp3)–H bond activation was investigated. It was found that the cyclization of enynes having a tert-butyl moiety on the alkene afforded a spirocyclic compound (up to 92% ee), while the cyclization of enynes having an isopropyl or an ethyl group on the alkene gave a cyclic diene (up to 98% ee). Furthermore, an intermolecular competition reaction using a deuterium-labeled substrate revealed that C(sp3)–H bond activation was one of the key steps, having a high energy barrier, in this cyclization.

β-Diazocarbonyl Compounds: Synthesis and their Rh(II)-Catalyzed 1,3 C-H Insertions

Herein, we describe the first electrophilic diazomethylation of ketone silyl enol ethers with diazomethyl-substituted hypervalent iodine reagents that gives access to unusual β-diazocarbonyl compounds. The potential of this unexplored class of diazo compounds for the development of new reactions was demonstrated by the discovery of a rare Rh-catalyzed intramolecular 1,3 C-H carbene insertion that led to complex cyclopropanes with excellent stereocontrol.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

A rhodium catalyzed cycloisomerization and tandem Diels-Alder reaction for facile access to diverse bicyclic and tricyclic heterocycles

A rhodium catalyzed cycloisomerization of 1,6-allenenes successfully prepared six-membered ring exocyclic 1,3-dienes. Further tandem Diels–Alder reaction with various dienophiles provided diverse bicyclic and tricyclic heterocycles. This methodology is characterised by complete atom and step economy, low catalyst loading, and excellent chemo-, regio-, and diastereoselectivity.

A regioselective distal cycloisomerization of 1,6-allenenes was successfully developed to afford six-membered ring exocyclic 1,3-dienes employing a rhodium/diphosphine catalyst system. Deuterium labelling experiments and DFT calculations were performed to provide insights into the reaction mechanism of this unprecedented transformation. In addition, one-pot tandem Diels–Alder reactions with various dienophiles could readily construct diverse bicyclic and tricyclic nitrogen heterocycles, which are ubiquitous core scaffolds for a variety of natural products and bioactives. High efficiency and exclusive chemo and regioselectivities for a broad substrate scope were achieved under mild conditions using a low catalyst loading of 0.5 mol%.

Key Mechanistic Features in Palladium-Catalyzed Methylcyclopropanation of Norbornenes With Vinyl Bromides: Insights From DFT Calculations

DFT calculations were performed to elucidate mechanistic details of an unusual palladium-catalyzed methylcyclopropanation from [2 + 1] cycloadditions of (Z)-2-bromovinylbenzene and endo-N-(p-tolyl)-norbornenesuccinimide. The reaction proceeds via oxidative addition (OA), intermolecular alkene insertion, deprotonation/protonation, intramolecular alkene insertion, β-H elimination and reductive elimination (RE). Protonation is the rate-limiting step and requires an overall barrier of 28.5 kcal/mol. The sources of two protons for protonation and exchange have also been clarified and the calculations agree with experimental observations.

Recent advances in the synthesis of cyclopropanes

Cyclopropanes, one of the most important strained rings, have gained much attention for more than a century because of their interesting and unique reactivity. They not only exist in many natural products, but have also been widely used in the fields of organic synthesis, medicinal chemistry and materials science as versatile building blocks. Based on the sustainable development in this area, this review mainly focuses on the recent advances in the synthesis of cyclopropanes classified by the type of catalytic system, including regio-, diastereo-, and enantio-selective reactions.

Enantioselective Rhodium-Catalyzed Cycloisomerization of 1,6-Allenynes to access 5/6-Fused Bicycle[4.3.0]nonadienes

Transition-metal-catalyzed cycloisomerization of 1,n-allenynes represents a powerful synthetic tool to rapidly assemble complex polycyclic skeletons from simple linear substrates. Nevertheless, there are no reports of the asymmetric version of these reactions. Moreover, most of these reactions proceed through a 6-endo-dig cyclization pathway, which preferentially delivers the distal product (via 5/5 rhodacyclic intermediate) rather than the proximal one (via 6/5 rhodacyclic intermediate). Herein, we report an enantioselective rhodium(I)-catalyzed cycloisomerization of 1,6-allenynes to provide the proximal product 5/6-fused bicycle[4.3.0]nonadienes in good yields and with excellent enantioselectivities. Remarkably, this chemistry works perfectly for 1,6-allenynes having a cyclic substituent within the allene component, thereby affording synthetically formidable tricyclic products with excellent enantioselectivities. Moreover, extensive DFT calculations suggest an uncommon pathway involving 5-exo-dig cycloisomerization, ring-expansion, rate-determining alkene isomerization involving C_sp3-H activation, C-C activation of the cyclobutene moiety and finally reductive elimination. Deuterium labeling experiments support the rate-determining step involving the C–H bond activation in this transformation.

Cycloisomerization of 1,n-allenynes allows to efficiently assembly polycyclic skeletons, however its asymmetric variant remains elusive. Here, the authors report the enantioselective Rh(I)-catalyzed cycloisomerization of 1,6-allenynes and disclose an uncommon 5-exo-dig pathway via DFT calculations.

Rhodium(I)-Catalyzed Cycloisomerization of Homopropargylallene-Alkynes through C(sp3 )-C(sp) Bond Activation

Upon exposure to a catalytic amount of [RhCl(CO)₂ ]₂ in 1,4-dioxane, homopropargylallene-alkynes underwent a novel cycloisomerization accompanied by the migration of the alkyne moiety of the homopropargyl functional group to produce six/five/five tricyclic compounds in good yields. A plausible mechanism was proposed on the basis of an experiment with ¹³ C-labeled substrate. The resulting tricyclic derivatives were further converted into the corresponding bicyclo[3.3.0] skeletons with vicinal cis dihydroxy groups.

Recent advances in the synthesis of cyclopropanes

Cyclopropanes have gained much attention by virtue of their interesting structure and unique reactivity. This review discusses the recent advances in the synthesis of cyclopropanes, and some of the related applications will be discussed.

Mechanistic Investigation of RhI-Catalyzed Cycloisomerization of Benzylallene-Internal Alkynes via C-H Activation

Treatment of the benzylallene-internal alkynes with [RhCl(CO)₂]₂ effected a cycloisomerization via a C_sp2-H bond activation to produce the tricyclo[9.4.0.0^3,8]pentadecapentaene skeleton. The reaction mechanism via formation of the rhodabicyclo[4.3.0] intermediates and σ-bond metathesis between the C_sp2-H bond on the benzene ring and the C_sp2-Rh^III bond was proposed. In addition, a plausible alternative mechanism for the previously reported cycloisomerization of the benzylallene-terminal alkynes could also be proposed.

Chemo- and Regioselective Rhodium(I)-Catalyzed [2+2+2] Cycloaddition of Allenynes with Alkynes

A highly chemo- and regioselective partially intramolecular rhodium(I)-catalyzed [2+2+2] cycloaddition of allenynes with alkynes is described. A range of diverse polysubstituted benzene derivatives could be synthesized in good to excellent yields, in which the allenynes served as synthetic equivalent to the diynes. A high regioselectivity could be observed when allenynes were treated with unsymmetrical alkynes.

Allenes, versatile unsaturated motifs in transition-metal-catalysed [2+2+2] cycloaddition reactions

ThisTutorial Reviewhighlights the versatility of allenes as unsaturated substrates in transition-metal-catalysed [2+2+2] cycloaddition reactions in generating complex architectures.

Palladium(0)-Catalyzed Methylcyclopropanation of Norbornenes with Vinyl Bromides and Mechanism Study

An unusual methylcyclopropanation from [2 + 1] cycloadditions of vinyl bromides to norbornenes catalyzed by Pd(OAc)2/PPh3 in the presence of CH3ONa and CH3OH has been established. A methylcyclopropane subunit was installed by a 3-fold domino procedure involving a key protonation course. Preliminary deuterium-labeling studies revealed that the proton came from methyl of CH3OH and also exposed an additional hydrogen/deuterium exchange process. These two proton-concerned reactions were fully chemoselective.

A rhodium(i)-catalysed formal intramolecular C–C/C–H bond metathesis

The rhodium-catalyzed phenylcyclobutane-to-indane transformation proceedsviaconsecutive chelation-assisted C–C bond cleavage and intramolecular C–H bond cleavage.

Electrophilic activation of allenenes and allenynes: analogies and differences between Brønsted and Lewis acid activation

This review focuses on electrophilic activation of allenes by an acid. Key mechanistic features are presented together with recent synthetic applications.

Palladium(0)-catalyzed cyclopropanation of benzyl bromides via C(sp3)–H bond activation

A novel Pd(0)-catalyzed domino cyclopropanation reaction was established. The process involves a Heck-type coupling reaction and a C(sp³)–H activation.